The present invention relates to image processing and, more particularly, to a method of representing a continuous tone image to reproduce image character and image color.
In traditional offset printing, a facsimile of the final document to be printed for the purpose of proofing may typically be created on a slow, high quality, low volume desktop device for customer approval. It is the intent of the offset press to match this proof. Such a proof is called a contract proof. Until recently, the resolution of the typical offset press greatly exceeded the resolution of most commercial xe2x80x9coff the shelfxe2x80x9d inkjet printers. Prior to the use of digital inkjet proofs, the contract proof may have been created using the exact film that would be used to image the plates for the offset run. By using similar ink, and similar media, the high quality proofing system could be made to match the offset press in both dot structure and color. The customer may sign the single copy produced by the proofer and attend the press run to make sure that the offset press output matched the proof prior to the start of mass production. Proof copies would typically not be created directly on the press due to the cost, waste, make-ready time and lost production time. The offset press is designed to make thousands or more copies while the high quality proofer would be optimized for creating just a single copy.
Present high quality inkjet proofers, on the order of 2400 or more dots per inch (dpi) have greatly reduced the time and expense involved in creating an offset proof. These high quality devices can now mimic the high quality achieved on an offset press. However, high speed digital inkjet presses print at a resolution lower than the desktop inkjet proofing devices. A proof of the continuous tone data produced by one of these devices exceeds the quality and character of output achievable on the digital press.
In order to better mimic the output of the digital press and to allow the digital press to meet or exceed the quality found on these proofs, the apparent resolution of these desktop proofing devices must be lowered to match that of the digital press. It is necessary to do this so that the contract proof will not overestimate the quality expectations of the digital press. This is most easily accomplished on proofing devices which have resolutions that are multiples of the digital printing press resolution. Merely reducing the resolution of the data is not sufficient to reproduce the printing artifacts of the digital press. For example, it is necessary to account for dot size within a pixel as well as the dot dispersion algorithm used to create the tone scale on the digital press. These factors contribute to the apparent xe2x80x9cgraininessxe2x80x9d of the digital press.
It is seen, therefore, that it would be desirable to define a method of image processing digital press data for the purpose of reproducing the image data on another device in such a way as to mimic both the color and the dot structure of the press output, without applying the stringent conditions of exact color match and dot structure of the press.
The present invention relaxes the stringency with which the proofing process must exactly match the source process in both resolution and frequency of dots. As long as the defining characteristics of the source process can be incorporated into the proofing process, the output can be made to closely resemble the source process, resulting in an accurate rendition of the digital press output.
In accordance with one aspect of the present invention, a method is provided for reproducing image data from a press output on another device. The present invention mimics color, resolution and approximate spatial frequency of dots characteristic of the source output. By allowing adjustments in both the spatial frequency (i.e., frequency modulation or FM) and the color intensity (i.e., amplitude modulation or AM) to vary from the original output device, it becomes possible to match the image artifacts, apparent resolution and color of the source device.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.